Cryogenic vacuum pumps (cryopumps) are widely used in high vacuum applications. Cryopumps are based on the principle of removing gases from a vacuum chamber by having them lose kinetic energy and then binding the gases on cold surfaces inside the pump. Cryocondensation, cryosorption and cryotrapping are the basic mechanisms that can be involved in the operation of the cryopump. In cryocondensation, gas molecules are condensed on previously condensed gas molecules. Thick layers of condensation can be formed, thereby pumping large quantities of gas.
Cryosorption is commonly used to pump gases that are difficult to condense at the normal operating temperatures of the cryopump. In this case, a sorbent material, such as activated charcoal, is attached to the coldest surface in the cryopump, typically the second stage cryoarray. The binding energy between gases and the adsorbing surface is greater than the binding energy between the gas particles themselves, thereby causing gas particles that cannot be condensed to adhere to the sorbent material and thus be removed from the vacuum system. When several monolayers of adsorbed gas have been built up, the effect of the adsorbing surface is lost and gas can no longer be pumped.
Cryopumps commonly have two stages. A two stage cryopump includes a first stage cryoarray, which typically operates at temperatures between 50K and 100K, and a second stage cryoarray, which typically operates at temperatures between 12K and 20K. A closed-loop helium refrigerator includes a two stage expander, which creates cryogenic refrigeration by the controlled expansion of compressed helium. The cryoarrays are thermally connected to the stages of the expander and are cooled by them.
Gases are pumped on three surfaces within the cryopump. The first stage cryoarray pumps gases, such as water vapor and carbon dioxide, at relatively high temperatures. These gases are pumped by cryocondensation. The top outside surface of the second stage cryoarray pumps gases, such as nitrogen, oxygen and argon, at the normal operating temperature of the second stage. The inside surfaces of the second stage cryoarray are coated with a sorbent material and pump the noncondensible gases hydrogen, neon and helium by cryosorption.
Under normal operating pressures, conditions of molecular flow exists in the cryopump. Practically all molecules entering the pump will strike the first stage cryoarray and the outside of the second stage cryoarray before reaching the sorbent material. Thus, all gases except hydrogen, neon and helium are pumped before reaching the sorbent material, keeping it free to pump those gases.
Finite amounts of gas can be accumulated on the pump surfaces before performance deteriorates and eventually becomes unacceptable. At this point, captured gases need to be released and expelled from the cryopump, thereby renewing the pumping surfaces for further service. This process, called regeneration, includes warming the cryopump until the captured gases evaporate. The gases are then removed from the cryopump through a pressure relief valve and/or are removed by a roughing pump. The cryopump is then cooled to its operating temperature and normal operation is resumed.
A key to optimum regeneration is to prevent the sorbent material used on the second stage cryoarray from becoming contaminated with previously pumped gases. A standard method for removing all captured gases, including condensed water vapor, without contaminating the sorbent material includes warming the cryopump to room temperature while purging it with a dry inert gas. To ensure that the sorbent material is not contaminated, the cryopump is purged for some time after reaching room temperature and then is pumped with a roughing pump. Since all captured gases are removed from the cryopump, this process is called full regeneration. Full regeneration typically takes more than two hours. During this time, the cryopump and the equipment to which it is attached are not operable.
To shorten regeneration time, a process called partial regeneration has been developed. In partial regeneration, only the gases pumped on the second stage cryoarray are removed. The cryoarrays are warmed to a temperature of 110K to 160K by flowing an inert gas, such as dry nitrogen, through the pump, by shutting off the refrigerator and/or by using electrical heaters. In this temperature range, all gases pumped on the second stage liquefy and evaporate. However, little or no condensed water vapor evaporates at these temperatures. One method for removing liquid and gas from the pump is through a one way valve, as described in PCT Publication Nos. WO92/05294 and WO92/08894. The cryopump is then pumped with a roughing pump and is cooled to normal operating temperature. Partial regeneration takes approximately 45 minutes.
The prior art partial regeneration process has difficulties. In order to complete partial regeneration in 45 minutes, the accumulated gas must be removed in less than 15 minutes. In many applications, such as sputtering, more than 500 standard liters of gas have been accumulated in the cryopump. Thus, the rate of gas removal must be high. This means that the pressure in the cryopump must be high. Conditions of viscous flow exist for several minutes, and large amounts of condensible gas, such as argon, nitrogen and oxygen, may reach the sorbent material under these conditions and be partially adsorbed. The amount of gas adsorbed depends on the type of gas, its pressure and the temperature of the sorbent material. For optimum results, the cryopump must be pumped to a low pressure while the sorbent material is at a relatively high temperature (greater than 120K), to minimize the amount of condensible gas remaining on the sorbent material when the pump is cooled down. Typically, the cryopump is pumped with a trapped rotary oil-sealed pump or a dry roughing pump. The speed of these pumps becomes low at pressures below 0.1 torr, particularly when connected to the cryopump through a roughing line of typical length and diameter. Also, the ultimate pressure of these roughing pumps is high, typically 10.sup.-3 to 10.sup.-4 torr, for the gases that are to be removed. This results in the potential for significant contamination of the sorbent material, and as much as 5% of its capacity can be lost per regeneration cycle.
The use of a roughing pump for the final stage of gas removal has disadvantages. Due to the low speed of the roughing pump at low pressures, it is difficult to remove the gas in a short time interval. Also, during gas removal, cryoarray temperatures must remain between 110K and 160K. It is difficult to maintain the heat flow balance in the cryopump for extended periods to keep cryoarray temperatures within these required limits.